The present application relates to a memory element including a memory layer that stores a magnetization state of a ferromagnetic layer as information and a magnetization pinned layer in which a magnetization direction is pinned. Specifically, the present application relates to a memory element in which current is caused to flow in a direction perpendicular to a film surface to inject spin-polarized electrons, so that a magnetization direction of the memory layer is changed; and also relates to a memory including the memory element suitably applied to a non-volatile memory.
High-speed and high-density DRAMs have been widely used as random access memories in information equipment such as computers.
However, since DRAMs are volatile memories from which information is erased when a power supply is switched off, non-volatile memories in which information is retained when a power supply is switched off have been desired.
Nikkei Electronics (Feb. 12, 2001 (pp. 164-171)), for example, discloses magnetic random access memories (MRAMs) that record information by magnetization of a magnetic material have been attracted attention and developed as potential non-volatile memories.
In an MRAM, the information is recorded such that current is caused to flow into two types of address wirings almost perpendicular to each other (word lines and bit lines), respectively, to invert magnetization of a magnetic layer of a magnetic memory element in an intersection of the address wirings based on a current magnetic field generated from each address wiring.
A schematic view (oblique view) of a typical MRAM is shown in FIG. 1.
In an area isolated by an element isolation layer 102 of a semiconductor substrate 110 such as a silicon substrate, a drain region 108, source regions 107, and gate electrodes 101 which form selective transistors for selecting each memory cell are respectively formed.
Word lines 105 extending in a longitudinal direction in the figure are provided above the gate electrodes 101.
The drain region 108 is also formed both on the left and right selective transistors in the figure, and a wiring 109 is connected to the drain region 108.
Magnetic memory elements 103 each having a memory layer in which a magnetization direction is inverted are placed between the word lines 105 and bit lines 106 that are placed above the word lines 105 and extended in a transverse direction in the figure. The magnetic memory elements 103 are formed by magnetic tunnel junction elements (MTJ elements), for example.
Further, the magnetic memory elements 103 are electrically connected to the source regions 107 through a bypass line 111 in a horizontal direction and a contact layer 104 in a vertical direction.
Current is caused to flow into the word lines 105 and the bit lines 106, respectively, to apply a current magnetic field to the magnetic memory elements 103, so that a magnetization direction of the memory layers of the magnetic memory elements 103 may be inverted to record information.
In order to allow a magnetic memory such as MRAM to stably retain recorded information, a magnetic layer (memory layer) to record information preferably may have a certain coercive force.
On the other hand, in order to rewrite the recorded information, a certain amount of current may preferably be caused to flow into address wirings.
However, as an element forming an MRAM is decreasing in size, address wirings are thin, and thus a sufficient amount of current may not be caused to flow into the wirings.
Japanese Patent Application Publication No. 2003-17782, U.S. Pat. No. 6,256,223, Phys. Rev. B 54.9353 (1996), and J. Magn. Mat. 159.L1 (1996), for example, respectively disclose memories that configured to use magnetization inversion by spin injection have been attracted attention as those configured to allow magnetization to be inverted using a smaller amount of current.
In magnetization inversion by spin injection, electrons spin-polarized by passing through a magnetic material are injected into another magnetic material to invert magnetization in the other magnetic material.
For example, current is caused to flow into giant magnetoresistance elements (GMR elements) or magnetic tunnel junction elements (MTJ elements) in a direction perpendicular to a film surface of the elements, so that a magnetization direction of at least some of magnetic layers of the elements may be inverted.
Magnetization inversion by spin injection is advantageous in that magnetization may be inverted without increasing an amount of current even though an element is reduced in size.
FIGS. 8 and 9 show schematic views of a memory configured to utilize the above-described magnetization inversion by spin injection; FIG. 8 is an oblique view, and FIG. 9 is a sectional view.
In an area isolated by an element isolation layer 52 of a semiconductor substrate 60 such as a silicon substrate, a drain region 58, source regions 57, and gate electrodes 51 which form selective transistors for selecting each memory cell are respectively formed. Of these, the gate electrodes 51 also function as word lines extending in a longitudinal direction in FIG. 8.
The drain region 58 is also formed both on the left and right selective transistors in FIG. 8, and a wiring 59 is connected to the drain region 58.
Memory elements 53 each having a memory layer in which a magnetization direction is inverted by spin injection are placed between the source regions 57 and bit lines 56 that are placed above the source regions 57 and extended in a transverse direction in FIG. 8.
The memory elements 53 are formed by magnetic tunnel junction elements (MTJ elements), for example. Reference numerals 61 and 62 in the figure denote magnetic layers. One of the two magnetic layers 61 and 62 is a magnetization pinned layer in which a magnetization direction is pinned, and the other is a magnetization free layer in which a magnetization direction is changed, specifically, a memory layer.
The memory elements 53 are connected to the bit lines 56 and the source regions 57 respectively through upper or lower contact layers 54. Thus, a magnetization direction of the memory layer may be inverted by spin injection that causes current to flow into the memory elements 53.
Such a memory configured to utilize magnetization inversion by spin injection has a feature in that the memory can have a device structure more simplified as compared with a common MRAM shown in FIG. 1.
The memory configured to utilize magnetization inversion by spin injection is more advantageous than a common MRAM in which magnetization is inverted by an external magnetic field, because an amount of writing current is not increased although the elements are further reduced in size.
In an MRAM, write wirings (word lines and bit lines) are provided separate from memory elements, and information is written (recorded) based on a current magnetic field generated by causing current to flow into the write wirings. Thus, an amount of current which may be necessary for writing may be sufficiently caused to flow into the write wirings.
On the other hand, in a memory configured to utilize magnetization inversion by spin injection, spin injection is preferably performed by causing current to flow into a memory element to invert a magnetization direction of a memory layer.
Since information is written (recorded) by directly causing current to flow into the memory element in this manner, the memory element is connected to a selective transistor to form a memory cell in order to select a memory cell to be written. In this case, an amount of current caused to flow into the memory element is limited to the amount of current which may be caused to flow into the selective transistor (saturated current of the selective transistor).
Therefore, writing is preferably performed using current in an amount equal to or smaller than the saturated current of the selective transistor, and an amount of current caused to flow into the memory element is preferably reduced by improving spin injection efficiency.
In order to amplify a read signal, a high magnetoresistance change rate may preferably be secured. To secure a high magnetoresistance change rate, it is effective to provide a memory element having a configuration in which an intermediate layer in contact with both sides of the memory layer forms a tunnel insulating layer (tunnel barrier layer).
When the tunnel insulating layer is used as an intermediate layer in this manner, an amount of current caused to flow into the memory element is limited in order to prevent dielectric breakdown of the tunnel insulating layer. From this viewpoint, an amount of current during spin injection may preferably be suppressed.
A memory preferably stores and retains information written by current. Thus, it is preferable to secure that a memory layer has stability against thermal fluctuation (thermal stability).
A memory element utilizing magnetization inversion by spin injection has a memory layer having its volume smaller than that of a memory layer of an MRAM of the related art. Thus, the memory element may tend to have decreased thermal stability.
When thermal stability of the memory layer is not secured, an inverted magnetization direction is inverted by heat, thereby causing a writing error.
Therefore, thermal stability is a highly important property in the memory element utilizing magnetization inversion by spin injection.
Generally, in an element that does not consume a large amount of energy in writing, an energy barrier is low and thus information is easily erased.
On the other hand, an element that consumes a large amount of energy in writing can form a high energy barrier and thus may stably retain information.
When comparing memory elements that utilize magnetization inversion by spin injection and are configured to have equal spin injection efficiency, such a memory element that has a memory layer having a larger amount of saturated magnetization and a larger volume has higher thermal stability and, at the same time, may need a larger amount of current in writing.
A thermal stability index can generally be represented by a thermal stability parameter (Δ).
The thermal stability parameter (Δ) is obtained from the formula Δ=KV/kT (K: anisotropic energy, V: volume of the memory layer, k: Boltzmann constant, T: temperature).
Accordingly, in order that a memory may be formed by a memory element configured to have a memory layer in which a magnetization direction is inverted by spin injection, it is preferable that spin injection efficiency be improved so that an amount of current is reduced, which may be necessary for magnetization inversion to equal to or smaller than saturated current of a transistor, and thermal stability be secured to stably retain written information.